Light biased photoresponsive array having low conductivity regions separating individual cells

ABSTRACT

A high density planar array of substrate-supported, thin film diodes constituting photoresponsive semiconductor elements is placed between a document to be read and a light source for illuminating the document. The document is illuminated by the light source via windows in the array, reflected light from the document impinging on the photoresponsive elements constituted by the diodes which are electronically interrogated to provide a signal which is indicative of image information on the document. The light source directly illuminates small portions of each diode to establish a discharge or bias current that makes the array more sensitive and responsive. The high density diodes of the array can be formed in a manner that minimizes leakage current between adjacent diodes.

This application is a division of application Ser. No. 07/339,063, filedApr. 17, 1989, which is a continuation-in-part application of copendingU.S. patent application Ser. No. 224,841 filed July 26, 1988, which is adivisional application of U.S. patent application Ser. No. 40,532 filedApr. 17, 1987, now U.S. Pat. No. 4,785,191. Pending U.S. patentapplication Ser. No. 224,841 and U.S. Pat. No. 4,785,191, both owned bythe assignee of the present application, are incorporated by referenceherein in their entireties.

BACKGROUND OF THE INVENTION

This invention relates in general to a photoresponsive array ofsemiconductor elements for electronically detecting or reading lightcontrasting images provided by a medium such as a document. Morespecifically, the present invention is directed to improving theperformance of a high density photodiode array of the thin film typethat is placed between a document to be read or scanned by the array anda light source for illuminating the document.

Light from the light source passes through transparent window areas inthe array and impinges on the document which is in close proximity to orin contact with the array. The reflected light from the documentilluminates the photodiodes of the array, which are then electronicallyinterrogated or read during a scan to provide a signal indicative of theimages on the document. The spatial density of the photodiodesdetermines the optical resolution of the array, each diode constitutinga pixel element. The array can be in the form of a line scanner or afull page scanner.

Such arrays are generally known in the art as thin film backlit arrayswhich rely on proximity focusing, i.e., the reflected light from thedocument being read impinges on the photodiodes of the array withoutpassing through any focusing elements such as collimators, lenses or thelike.

U.S. Pat. Nos. 4,149,197 and 4,660,095 illustrate thin film, backlitarrays of the proximity focusing type wherein the respective inventorsteach that the photoresponsive elements or light sensors of theirrespective arrays must be shielded from direct impingement by light fromthe light source in order to avoid light saturation of the associatedlight sensors.

SUMMARY OF THE INVENTION

In accordance with the present invention, a substrate is provided andsupports an array of photoresponsive semiconductor elements. The arrayis positionable adjacent to a medium presenting light contrasting imagesto be read by the array. Light from an illumination source is projected,e.g., reflected from or transmitted through, the medium onto thesemiconductor elements. The semiconductor elements are thenelectronically read during a scan to provide an electrical signalindicative of the images presented by the medium. Improved performanceof the array is provided by directing some of the light from theillumination source directly onto a limited portion of eachphotoresponsive semiconductor element to generate a current fordischarging any residual charges on the semiconductor elements (e.g.,caused by parasitic capacitance) before the next scan. This current canalso constitute a reference or bias part of the electrical signalprovided by the array as it is read. Such a feature provides a moresensitive and responsive array as compared to those of the prior art.

Preferably the semiconductor elements are thin film photodiodes and thereference part of the signal is a discharge or bias current generated bythe photodiodes in response to being directly illuminated by light fromthe light source not affected by the medium. The medium can be in theform of a printed document, for example, that is to be photocopied ortransmitted by facsimile. The substrate can be generally planar, thusproviding two sides, the photodiodes being carried on one side of thesubstrate. Each photodiode has an upper surface upon which impingeslight reflected or transmitted through the medium or document. Remainingsurfaces of the diode provide the limited portion upon which directlyimpinges light from the light source that has not been affected by themedium or document, i.e., not affected by the light contrasting imagesto be read.

In the embodiment of the invention illustrated herein, the array ispositionable adjacent to the medium or document, the array including atleast one window area through which light from the illumination sourcepasses. The light passes through one or more window areas and isreflected from the medium or document onto the photodiode elements. Thesubstrate of the array can be a transparent piece of glass having anunderside adjacent to the illumination source and an upper side carryingthe photodiode elements which are preferably monolithically formed onthe substrate. The medium or document is placed in close proximity to oron the upper surface of the array. A small portion of the light istransmitted through the transparent substrate directly into the activeregion of the diodes to establish the above-noted discharge or biascurrent.

In further accordance with the present invention, the photodiodes, whichare closely spaced together (on the order of 10 microns) to provide thedesired optical or pixel density, are electrically isolated from eachother to a degree to minimize inter-diode leakage currents or cross talkby removing some semiconductor material between adjacent diodes. Theremoved semiconductor material can be replaced by a less electricallyconductive material if desired.

Arrays provided in accordance with the present invention, while simplein design and manufacturing process, have been found to be especiallyuseful in providing fast response, high definition arrays for both blackand white and gray scale indicative image detection.

BRIEF DESCRIPTION OF THE DRAWINGS

A fuller understanding of the invention may be had by referring to thefollowing description and claims taken in conjunction with theaccompanying drawings, wherein:

FIG. 1 is a schematic representation of a portion of a photoresponsivearray in accordance with the present invention;

FIGS. 1A and 1B are graphical representations of possible output signalsof the array portion illustrated in FIG. 1;

FIG. 2 is a non-scale, perspective view of the physical structure of thearray portion schematically illustrated in FIG. 1;

FIG. 3 is an exploded view of the array portion illustrated in FIG. 2;

FIGS. 4 and 5 are cross section views of portions of FIG. 2 taken alonglines 4--4 and 5--5, respectively;

FIGS. 6 and 6A illustrate alternatives or modifications to the arraystructure illustrated in FIG. 5; and

FIG. 7 is an illustration of the array structure of FIG. 4 positionedbetween an illumination source and the medium or document to be read bythe array of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically illustrates a portion of a photoresponsive array 10which includes a plurality of photoresponsive semiconductor elements,namely first, second and third photodiodes 20, 30, 40. The array 10 alsoincludes another plurality of non-photoresponsive semiconductorelements, namely first, second and third non-photoresponsive orconventional diodes 50, 60, 70.

The photodiodes 20, 30, 40 each have respective anode electrodes 22, 32,42 and cathode electrodes 24, 34, 44 between which is sandwichedsemiconductor material 23, 33, 43 constituting the active regions of thephotodiodes. Appropriate anode leads 26, 36, 46 and cathode leads 28,38, 48 are provided for electrically connecting the photodiodes 20, 30,40 to other circuit elements.

In a similar fashion, non-photoresponsive or conventional diodes 50, 60,70 each have respective anode electrodes 52, 62, 72 and cathodeelectrodes 54, 64, 74 between which is sandwiched semiconductor material53, 63, 73 constituting the active regions of the non-photoresponsivediodes. Each of the diodes 50, 60, 70 provides respective anode leads56, 66, 76 and cathode leads 58, 68, 78 to permit electrical connectionof the diodes 50, 60, 70 to other circuit elements.

The anode leads 26, 36, 46 of the photodiodes are electrically connectedtogether via a bus bar 14 having an end constituting a first terminal 12of the array. The non-photoresponsive diodes 50, 60, 70 are electricallyconnected in a series relationship with each other as illustrated. Thecathode lead 48 of photodiode 40 is connected to the cathode lead 78 ofnon-photoresponsive diode 70 and to the anode lead 66 of diode 60, theanode lead 76 of diode 70 being connected to a second terminal 16 of thearray. In a similar fashion, cathode lead 38 of photodiode 30 iselectrically connected to cathode lead 68 and anode lead 56 ofnon-photoresponsive diodes 60 and 50, respectively. Similarly, thecathode lead 28 of photodiode 20 is electrically connected to cathodelead 50 of non-photoresponsive diode 50.

As will be recognized by those skilled in the art, thenon-photoresponsive diodes 50, 60, 70 are electrically connected in likepolarity series string circuit relationship wherein one end of thestring is the second terminal 16 of the photoresponsive array portion.The photodiodes 20, 30, 40 are electrically connected to a different oneof the electrical junctions between the non-photoresponsive diodes 50,60, 70.

The degree of illumination of each photodiode 20, 30, 40 is determinedby applying a ramp voltage signal to the array terminals 12, 16 and thenobserving the sequential current flows as the ramp voltage signalincreases. The changes in current flow, indicating various degrees ofillumination of the photodiodes, occur at ramp voltage signal pointsthat correspond to the physical positions of the photodiodes asdetermined by their sequential conduction. From the current flowsdetected during application of the ramp signal, the desired illuminationversus location information can be determined. In effect, the sequentialbreakover or forward conduction of non-photoresponsive diodes 50, 60, 70determines the location of the photodiodes 20, 30, 40. The degree ofcurrent conduction of the photodiodes 20, 30, 40 determines the level oflight intensity illumination of the individual photodiodes 20, 30, 40functioning as array pixel elements.

In accordance with the present invention, and contrary to the earlierdiscussed prior art, the photodiodes, 20, 30, 40 are partiallyilluminated, at least between scans, to establish a discharge current toaccelerate the discharge of the diodes 20, 30, 40 between scans, suchdischarging overcoming the negative effects of the parasitic capacitanceof the diodes 20, 30, 40. The discharge current can also function as abias current level which provides in some applications for optimizedlight detecting sensitivity of the photodiodes, particularly in blackand white, or "on" and "off" detection.

It is to be noted that while only three photodiode/non-photodiode pairsare illustrated, in practice up to 32 diode pairs in a string of thetype illustrated in FIG. 1 are provided. This is schematicallyillustrated by the dotted lines extending to the left of FIG. 1.

With reference to FIG. 1A, as an example of the response of the arrayportion of FIG. 1 to an interrogating voltage ramp signal, measuredresponses of the array portion incorporating the three discretephotodiodes 20, 30, 40 and three discrete non-photoresponsive diodes 50,60, 70 measured on a curve tracer are shown in FIGS. 1A and 1B. Theresults are indicated with respect to the photodiode 40 which is leastremote from the terminals 12, 16, the photodiode 30, next least remotefrom the network terminals, etc. In FIG. 1A, the photodiode 40 is notilluminated, while the other two photodiodes 30, 20 are illuminated. InFIG. 1B, only the photodiode 30 is not illuminated. Clearly, when aparticular photodiode is illuminated, there is a current change within avoltage range that discloses the position and illuminated condition ofthe interrogated photodiode. The information thus gained can beelectronically processed, stored, manipulated, and/or used to produce adesired result, e.g., an image on a page of paper, a display on amonitor, or stored data. The electronics may include a microprocessor tomanipulate the information in a particular, preselected manner.

As already mentioned, the voltage at which a step increase in currentoccurs corresponds to a position along the array. The amount of thecurrent increase corresponds to the light intensity at that position. Inthe absence of illumination, the step increase in current is very small,or non-existent. To improve array performance in accordance with oneaspect of the present invention as noted above, each photoresponsivediode is directly illuminated at an intensity that is below the lowestintensity signal intended to be detected. With that limited backgroundillumination, a small bias current step will be present at eachphotoresponsive diode in the array. That position information consistsin confirming the presence or absence of a detected signal at eachphotoresponsive diode in the array, as well as avoiding any undesirableresidual charge storage effects (caused, for example, by parasiticcapacitance of the diodes) that could reduce the response or speed ofthe array, i.e., the bias current also functions as a discharge currentto accelerate the discharge of the photodiodes between scans, as notedabove.

A more detailed discussion of the electrical operation and ofalternative embodiments to the schematically illustrated array portionof FIG. 1 are set forth in heretofore incorporated U.S. Pat. No.4,785,191 issued to the inventor of the present application.

FIG. 2 illustrates in perspective view that portion of thephotoresponsive array 10 schematically illustrated in FIG. 1. In FIG. 2,the reference numerals of FIG. 1 have been provided to identifycorresponding structural components of an actual photoresponsive arrayportion in accordance with the present invention. In a similar fashion,FIG. 3 is an exploded view of FIG. 2, and also provides referencenumerals corresponding to those of FIGS. 1 and 2.

With reference to FIGS. 2 and 3, which are not to scale, the arrayportion 10 includes a generally planar substrate 80 having an uppersurface or side 84 and a lower surface or side 86, as illustrated.Preferably, the substrate is formed from electrically non-conductive,light transparent material such as plate glass of high optical qualityhaving a thickness of, for example, one millimeter. The term"transparent" in the context of the present invention means that thesubstrate 80 is preferably transparent to the wavelength of the light orother electromagnetic radiation being detected, whether visible ornon-visible, such as infrared, for example.

The photoresponsive elements or sensors of the array, and relatedinterconnecting leads, are monolithically formed on the upper side 84 ofthe substrate 80. Such formation can be provided by use of conventionalphotolithography and material deposition tecniques, such aselectrodepositing and vapor deposition, to provide thin filmsemiconductor elements, as is well known in the art.

With specific reference to FIG. 3, a lower layer or cathode level ofelectrically conductive material includes the cathode electrodes 24, 34,44 in the form of generally square, rectangular pads, as illustrated.Extending from the pads are respective cathode leads 28, 38, 48 whichextend to meet associated cathode leads 58, 68, 78 of thenon-photoresponsive diodes 50, 60, 70 discussed earlier in reference toFIG. 1. The cathode leads 58, 68, 78 extend to their respective cathodeelectrodes 54, 64, 74, which are also formed in the shape of generallysquare, rectangular pads. Also provided along the top edge (asillustrated in FIG. 3) of the substrate 80, on the top surface 84thereof, is a lower bus bar portion 14b constituting a part of bus bar14 schematically illustrated in FIG. 1. The gridlike electrode and leadelements thus far discussed, constituting the lower layer or level ofthe photoresponsive sensor or array structure, are preferably formed ofan electrically conductive, vapor deposited metal, such as molybdenum,chromium, or aluminum, which is substantially opaque to light, forreasons to be subsequently discussed. The thickness of the elements ofthe metal layer is typically a thousand to a few thousand (e.g.,1000-3000) angstroms. Other light-opaque, electrically conductivematerials could also be used.

With the lower layer elements deposited in place in a prescribed patternas illustrated on the upper surface 84 of the substrate 80, a middlelayer or level of lightopaque semiconductor material 90, preferablyamorphous or polycrystalline silicon, or other thin film semiconductormaterial, is deposited onto the upper side 84 of the substrate so as tocover the lower level elements, i.e., lower bus bar portion 14b, cathodeelectrodes 24, 34, 44, cathode electrodes 54, 64, 74 and their variousinterconnecting leads. The thickness of the semiconductor layer is onthe order of several hundred nanometers (e.g., 750) so as to attenuatelight transmission therethrough wherein such layer 90 is generallyopaque to light.

By using suitable selective chemical etching and photolithographytechniques or the like, a plurality of generally square, rectangularwindow areas 100 are provided through the semiconductor material layer90, as illustrated. Also, a plurality of interconnection apertures 14dare provided along the top end or edge of the semiconductor materiallayer 90. In a similar fashion, a plurality of interconnection apertures68b, 78b and 76b are provided along the lower end or edge of thesemiconductor layer 90.

In a further stage of forming a photoresponsive array in accordance withthe invention, an upper layer or anode level of elements is depositedonto the etched semiconductor material layer 90 which overlies the lowerlayer or level of elements as discussed earlier. This upper level ofarray elements, having a thickness of about 1000 angstroms, for example,is formed from a light transparent, electrically conductive materialsuch as vapor deposited indium tin oxide or the like. Such a transparentmaterial is used to allow light to transmit therethrough and impinge onthose portions of the semiconductor material layer 90 that are coveredby such upper layer or cathode level elements. Such a layer also reducesundesired light reflection from the upper level of array elements. Moreparticularly, the upper layer or anode level of elements includes anupper bus bar portion 14a which is connected to the lower bus barportion 14b via a plurality of integral interconnects or bridge portions14c extending through the interconnection apertures 14d, the bus barportion 14a and bridge portions 14c being simultaneously formed of thesame light transparent material. Extending from the upper bus barportion 14a are the anode leads 26, 36, 46 of the photoresponsive diodes20, 30, 40 (see FIG. 1). These leads 26, 36, 46 in turn extend to theanode electrodes 22, 32, 42 of the photodiodes. These anode electrodes22, 32, 42 are centered over and above the cathode electrodes 24, 34,44, and are of a square, rectangular shape geometrically identical tothe earlier discussed cathode electrodes 24, 34, 44, but have, inaccordance with the present invention, a smaller surface area dimensionthan such light-opaque cathode electrodes 24, 34, 44. The purpose ofthis difference in surface area will be discussed subsequently.

The upper level of elements also includes square, rectangle-shaped anodeelectrodes 52, 62, 72 forming part of the non-photoresponsive diodes 50,60, 70 (see FIG. 1), the anode electrodes 52, 62, 72 being centered overand above their respective cathode electrodes 54, 64, 74. The anodeelectrodes 52, 62, 72 have related anode leads 56, 66, 76. Anode leads56, 66 are connected to lower level elements of the array (i.e., cathodeleads 68, 78) by respective interconnects or bridge portions 78a and 76awhich extend through the interconnection aperture 78b and 76b, asillustrated most clearly in FIG. 3. Another interconnect or bridgeportion 68a extends through aperture 68b to provide for the connectionof leads 58 and 28 to an adjacent diode (not shown). It can be seen thatthe exploded structure of FIG. 3, shown in an assembled form in FIG. 2,electrically provides the circuit illustrated and discussed earlier withregard to FIG. 1. Diodes 50, 60, 70 are like polarity, seriesrelationship with each other, while photodiodes 20, 30, 40 areelectrically connected to appropriate inter-diode junctions in thestring of non-photoresponsive diodes 50, 60, 70 as discussed earlier.

With particular reference to FIG. 2, as will be recognized by those inthe art, the photoresponsive diodes or photodiodes 20, 30, 40 (seeFIG. 1) are constituted by those portions of the semiconductor materiallayer 90 sandwiched between electrode pairs 22-24, 32-34, and 42-44. Ina similar fashion, the non-photoresponsive or conventional diodes 50,60, 70 (see FIG. 1) are constituted by those portions of thesemiconductor layer 90 sandwiched between electrode pairs 52-54, 62-64,72-74.

The photodiodes 20, 30, 40 are spaced closely together (approximately 10microns apart) and aligned, each photodiode being adjacent to a windowarea 100 as illustrated. As will be illustrated in greater detailsubsequently, light will shine through the transparent substrate 80 andthen through the window areas 100 to impinge on an overlying lightcontrasting medium, such as a document, the light reflecting off thedocument and down into the active region of the diodes 20, 30, 40 viatransparent anode electrodes 22, 32, 42. It should be noted that otherwindow/diode configurations are contemplated. For example, two portionsof one diode, e.g., along opposed sides of one window, could beprovided. Also, annular or ringlike windows surrounding the associateddiode could be provided. Multiple windows could also be provided foreach diode.

By design, the non-photoresponsive diodes having active regionsconstituted by portions of the semiconductor layer 90 sandwiched betweenelectrode pairs 52-54, 62-64, 72-74 are spaced far enough away from thewindow areas 100 so as not to be illuminated by light transmittedtherethrough. It is contemplated that conventional light-opaqueshielding could be provided to mask the non-photoresponsive diodes tothe extent required. As noted earlier, generally light-opaquesemiconductor material layer 90 is of a required thickness so that it ineffect masks or shields an overlying document portion from illuminationfrom a light source other than by light passing through the transparentsubstrate 80 via the window areas 100.

Turning to FIG. 4, a cross section view along line 4--4 of FIG. 2 isillustrated. In a similar fashion with reference to FIG. 5, a crosssection view along line 5--5 of FIG. 2 is illustrated. As shown in FIG.4, one of the window areas 100 is adjacent to an edge of its respectivephotodiode 30 constituted by its padlike anode electrode 32 and itspadlike cathode electrode 34. The active region of the diode 30, i.e.,that area of the semiconductor material layer 90 located only betweenthe surface areas of electrodes 32, 34, is, for example in the form of ap-i-n diode structure. An n-i-p diode structure could also be used. Thelayer 90 can be sequentially deposited to provide a thin p-doped layer,a relatively thick intrinsic layer or i layer, and a thin n-doped layer,as illustrated. With reference to both FIGS. 4 and 5, and in accordancewith the present invention, the dimensional surface area of thetransparent anode electrode 32 is smaller than the surface area of therelated cathode electrode 34. More specifically, the distance Dconstituting the width or length of the generally square, rectangularpadlike cathode electrode 34 is greater than the distance d or width orlength of the generally square, rectangular padlike anode electrode 32,as illustrated in both FIGS. 4 and 5. This is important so that thelight-opaque cathode electrode 34 substantially shield the activeperipheral region of the diode from the light being transmitted throughthe transparent substrate 80. It will be recognized that the smallersurface area of the electrode 32 generally defines the peripheral limitsof the active region of the diode which are overlapped by the largersurface area cathode electrode 34. While the material of layer 90 hasbeen termed "generally" light-opaque, it is recognized that lighttransmitted through the substrate 80 can to a limited degree penetratethe underside of the layer 90, and that this is an important feature ofthe invention, as will become apparent.

With further reference to FIG. 4, the anode lead 36 can be seen toextend from the anode electrode 32. It will be recognized that the anodelead 36 extends vertically over the lower edge of the larger cathodeelectrode 34 as illustrated by arrow 35. In other words, the surfacearea of the larger cathode electrode 34 extends horizontally beyond thesurface area of the overlying anode electrode 32 but for the area wherethe anode lead 36 joins to the anode electrode 32. Thus, the activeregion of the diode 30 includes that small part of semiconductormaterial sandwiched between the leftward end of the anode lead 36 asillustrated in FIG. 4, and right edge of the cathode electrode 34, thissmall portion of the active area of the diode constituting the regionindicated by circled area 35a. Similar regions are provided for diodes20 and 40 and all other photodiodes in the array. This region 35aprovides the earlier-noted discharge or bias current for the diode sinceit can, to a limited degree, be directly illuminated by an illuminationsource, as will be subsequently illustrated. Thus, as shown in FIG. 5,the lower electrodes 24, 34, 44 are of a larger surface area so as tosubstantially shield the active regions 23, 33, 43 defined by thesmaller overlying and above centered anode electrodes 22, 32, 42, butfor those regions directly between anode leads 26, 36, 46 and underlyingcathode electrodes 24, 34, 44. It is contemplated that other electrodegeometries such as circular, non-square, etc. could provide the featuresnoted above. It is also contemplated that the non-photoresponsive diodes50, 60, 70 may be formed with a geometry to provide the current biasingfeature (like photoresponsive diodes 20, 30, 40), although this is notrequired.

A modification of the structure illustrated in FIG. 5 is shown in FIGS.6 and 6a. To minimize leakage current or cross talk between adjacentdiodes, the conductivity of the semiconductor material layer 90 is veryclosely controlled and is reduced to the lowest level practical. Incases where leakage currents or cross talk between the closely spaced,adjacent diodes illustrated in FIG. 5 are still at an undesirable leveldue to the close proximity (10 microns) of the diodes and due to thepresence of the semiconductor material therebetween, linear recesses 120can be provided (e.g., by etching) between the anode electrodes 22, 32,42 so as to remove some of the semiconductor material layer 90 frombetween the diodes. In accordance with the present invention, such afeature inherently minimizes the flow of leakage currents between thediodes as will be appreciated by those skilled in the art. In furtheraccordance with the present invention, a non-electrically conductingmaterial 121 can be used to fill in the recesses 120 of FIG. 6 and can,if transparent, also be of a thickness to cover, as a thin protectivelayer, the top surfaces of the transparent anode electrodes 22, 32, 42so that they do not come in direct contact with the light contrastingmedium or document contiguous thereto that is being read. For example, asuitable material 121 (see FIG. 6A) could be silicon nitride,diamondlike-microcrystalline carbon or other abrasion-resistant,transparent material, as taught by U.S. Pat. No. 4,691,243.

Turning to FIG. 7, a portion of the array 10, as discussed earlier withparticular regard to FIG. 4, is illustrated in position between a mediumsuch as a document 130 carrying on its underside light contrastingimages to be detected. The upper surface of the diodes is adjacent tothe underside of the document 130, while the lower and side surfaces ofthe diode are closer to the upper surface 84 of the substrate 80.

A light source 140 is provided adjacent to the underside 86 of thesubstrate 80 wherein light (see arrows 142) is transmitted, via thewindow area 100, through the transparent substrate 80 and impinges (atleast a portion thereof) on the document 130. Such light, some of itbeing absorbed by the document 130, and light contrasting images carriedthereon are reflected down into the active region of the adjacent diodeto affect the amount of current flow therethrough as discussed earlier.In accordance with the invention, some of the light (see arrow 144) istransmitted through the transparent substrate 80 and penetrates thelower level of the generally light-opaque semiconductor layer 90 so asto impinge directly on a small part or lower end of the side region ofthe diode (circles area 35a) as discussed earlier with regard to FIG. 4.The penetration of the light into the active region is of a very smallproportion relative to the amount of light impinging on the top surfaceof the diode via optical window 100. However, the light impingement(arrow 144) directly on the diode does establish the earlier discusseddischarge or bias current that permits good contrast discrimination, aswell as providing a discharge path for non-illuminated diodes, i.e.,diodes adjacent to black or dark areas on the document 130. Betweenscans, the current induced by direct lighting (arrow 144) of the diodeallows complete and rapid discharge of the diode so as to avoid residualcharges caused by parasitic capacitance. It is to be noted that othermeans for providing the bias current are contemplated. For example, asmall hole through the lower cathode electrode 34 could allow partialillumination of the underside of the diode to provide the bias current.

By use of the diode structure providing the diode discharge bias currentas noted, and in cases where minimum inter-diode leakage current isrequired as discussed with regard to FIGS. 6 and 6A, a simple andefficient array structure has been provided.

It should be evident that this disclosure is by way of example and thatvarious changes may be made by adding, modifying or eliminating detailswithout departing from the scope of the teaching contained in thisdisclosure. For example, light could be transmitted through the document130 (see FIG. 7) and then be projected onto the underlying array, thewindows 100 being unnecessary, and a separate light source providing thelight indicated by arrow 144. Also, the diode array layers could bereversed wherein the anode electrode layer would be formed oflight-opaque conductive metal, while the lower cathode layer could beformed of light transparent conductive material, such as indium tinoxide. The invention is therefore not limited to particular details ofthis disclosure except to the extent that the following claims arenecessarily so limited.

What is claimed is:
 1. In a device including a substrate supporting anarray of photoresponsive semiconductor diode elements spaced closelytogether, the array being positionable adjacent to a medium presentinglight contrasting images to be read by the array, the semiconductorelements being electronically read to provide an electrical signalindicative of the images presented by the medium, each semiconductorelement including a portion of a semiconductor material layer sandwichedbetween an upper electrode and a lower electrode, the improvementwherein at least a portion of the semiconductor material layer betweenadjacent diodes is removed and replaced by a less electricallyconductive material so as to minimize leakage current flow or cross-talktherebetween.
 2. A device according to claim 1, wherein said lesselectrically conductive material also extends over one of saidelectrodes.